Process for NiFe fluxgate device

ABSTRACT

An etchant for simultaneously etching NiFe and AlN with approximately equal etch rates that comprises phosphoric acid, acetic acid, nitric acid and deionized water. Alternating layers of NiFe and AlN may be used to form a magnetic core of a fluxgate magnetometer in an integrated circuit. The wet etch provides a good etch rate of the alternating layers with good dimensional control and with a good resulting magnetic core profile. The alternating layers of NiFe and AlN may be encapsulated with a stress relief layer. A resist pattern may be used to define the magnetic core geometry. The overetch time of the wet etch may be controlled so that the magnetic core pattern extends at least 1.5 um beyond the base of the magnetic core post etch. The photo mask used to form the resist pattern may also be used to form a stress relief etch pattern.

FIELD OF THE INVENTION

This invention relates to the field of integrated circuits. Moreparticularly, this invention relates to fluxgate magnetometers inintegrated circuits.

BACKGROUND OF THE INVENTION

Some integrated circuits have a fluxgate magnetometer. A fluxgatemagnetometer consists of a small, magnetically susceptible core wrappedby two coils of wire. An alternating electrical current is passedthrough one coil driving the coil through alternating cycles of magneticsaturation. The constantly reversing magnetic field in the core inducesan electrical current in the second coil. In a magnetically neutralbackground, the input and output currents match. However, when the coreis exposed to a background magnetic field, it will be more easilysaturated in alignment with that field and less easily saturated inopposition to it. Hence the alternating magnetic field, and the inducedoutput current, will be out of step with the input current. The extentto which they are out of step depends upon the strength of thebackground magnetic field. Typically the current in the output coil isintegrated yielding an output analogy voltage which is proportional tothe magnetic field.

Integrating the fluxgate magnetometer into the integrated circuitfabrication process requires forming a magnetic core of highpermeability material such as permalloy (NiFe), with a thickness of amicron or more. To improve electrical properties the magnetic core maybe formed of multiple, alternating layers of high permeability materialand a dielectric material such as aluminium nitride (AlN). Finding anetch that etches a thick stack of multiple layers of two differentmaterials with good dimensional control, with good profiles, and withinsensitivity to material properties such as grain size is challenging.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentsome concepts of the invention in a simplified form as a prelude to amore detailed description that is presented later.

An etchant for the simultaneous etching of NiFe and AlN with the sameetching rate comprised of phosphoric acid, acetic acid, and nitric acid.

A method of forming a fluxgate magnetometer in an integrated circuitincludes forming a magnetic core of the fluxgate magnetometer fromalternating layers of permalloy and AlN dielectric. A wet etchcontaining phosphoric acid, acetic acid, nitric acid, and deionizedwater provides a good etch rate of the alternating layers with gooddimensional control and with a good resulting magnetic core profile.

If desired, the alternating layers of NiFe and AlN may be encapsulatedwith a stress relief layer. A magnetic core photo resist pattern may beused to define the magnetic core geometry. The overetch time of the wetetch may be controlled so that the magnetic core pattern extends atleast 1.5 um beyond the base of the magnetic core post etch. The photomask used to form the magnetic core photo resist pattern may be used toform the stress relief material etch pattern.

DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1 is a cross section of an example integrated circuit containing afluxgate magnetometer.

FIG. 2 is a diagram of a fluxgate magnetometer.

FIG. 3A through FIG. 3D are cross sections of the integrated circuit ofFIG. 1 depicted in successive stages of fabrication.

FIG. 4 is a flow diagram describing a procedure for etching the magneticcore with the wet etchant.

FIG. 5 is a cross section of a magnetic core encapsulated with a stressrelief layer.

FIG. 6A through FIG. 6C are cross sections of the integrated circuit ofFIG. 5 depicted in successive stages of fabrication.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The following patent application is related and hereby incorporated byreference: US patent application Ser. No. 14/557,611, filedsimultaneously with this application. With its mention in this section,this patent application is not admitted to be prior art with respect tothe present invention.

The present invention is described with reference to the attachedfigures. The figures are not drawn to scale and they are provided merelyto illustrate the invention. Several aspects of the invention aredescribed below with reference to example applications for illustration.It should be understood that numerous specific details, relationships,and methods are set forth to provide an understanding of the invention.One skilled in the relevant art, however, will readily recognize thatthe invention can be practiced without one or more of the specificdetails or with other methods. In other instances, well-known structuresor operations are not shown in detail to avoid obscuring the invention.The present invention is not limited by the illustrated ordering of actsor events, as some acts may occur in different orders and/orconcurrently with other acts or events. Furthermore, not all illustratedacts or events are required to implement a methodology in accordancewith the present invention.

An integrated circuit with a fluxgate magnetometer may be formed with amagnetic core composed of multiple, alternating layers of NiFe permalloy(NiFe) and AlN dielectric. The AlN layers between the layers of NiFelayers improve the performance of the magnetometer by reducing eddycurrent losses at high frequencies. The magnetic core may be composed ofmultiple alternating layers of NiFe and AlN with a thickness of 1 micronor more. The magnetic core may be composed of about 3 to 10 layers ofthe NiFe/AlN laminate. A good etched profile is difficult to achievebecause of the difference in etching rate of NiFe and AlN in mostetchants. A good profile is important to prevent voids between themagnetic core and overlying dielectric that might lead to delaminationand circuit failure and also to reduce noise in the fluxgatemagnetometer which may limit the sensitivity for the detection of weakmagnetic fields. An etch has been developed that etches the multilayeredstack of AlN and NiFe with an acceptable etch rate, with gooddimensional control and with a good profile. The etch is insensitive tomaterial properties such as grain size that may vary across the wafer,may vary from wafer-to-wafer, and may vary from lot-to-lot.

FIG. 1 is a cross section of an example integrated circuit 100containing a fluxgate magnetometer 111. The fluxgate magnetometer 111 iscomprised of a magnetic core 120 which is surrounded by a coil ormultiple coils. The coil or coils are formed of a first set of metallines 108 under the magnetic core 120 and a second set of metal lines130 over the magnetic core that are coupled together with a first set ofvias 213 (FIG. 2) that lie in front of the magnetic core 120 and asecond set of vias 217 (FIG. 2) that lie behind the magnetic core 120.Vias 132 may also be used to form an electrical connection between firstmetal lines 104 to second metal lines 126. An underlying layer ofdielectric 110 electrically isolates the magnetic core 120 from thefirst stet of metal lines 108. An overlying layer of dielectric 124covers the sides and top of the magnetic core 120 and electricallyisolates it from the vias 132, 213, and 217, and also electricallyisolates is from the second set of metal lines 130. Although only oneflux gate magnetometer sensor coil is shown, typically there may be twoor more sensor coils.

The integrated circuit 100 includes a first interlevel dielectric (ILD)layer 102 which may include silicon dioxide-based material, such asorganosilicate glass (OSG), silicon nitride, silicon oxynitride, and/ora low dielectric constant (low-k) dielectric. A plurality of first metallines 104 having copper damascene structures are disposed in the firstILD layer 102, extending to a top surface 105 of the first ILD layer102. Each instance of the first metal line 104 includes a refractorymetal liner (not shown) of tantalum and/or tantalum nitride, and a fillmetal 106 of copper on the metal liner. One or more of the first metallines 104 may be connected to first vias 113 having copper damascenestructures, disposed in the first ILD layer 102. Other of the firstmetal lines 108 may be formed under the magnetic core and become part ofthe flux magnetometer sensor coils. The first metal lines 104 and firstvias 113 may be dual damascene structures as depicted in FIG. 1, or maybe single damascene structures.

An underlying dielectric layer 110 is disposed on the first ILD layer102 and on the first metal lines 104 and electrically isolates the firstmetal lines 104 from the magnetic core 120. The underlying dielectriclayer 110 may be 500 to 1000 nm thick. The underlying dielectric layer110 may include a first etch stop layer 112 over the first ILD layer 102and over the first metal lines 104. The first etch stop layer 112 may beprimarily silicon nitride-based dielectric material, 35 nanometers to150 nanometers thick, which advantageously reduces copper migration fromthe first metal lines 104. Dielectric layer 114 which is formed on thefirst etch stop layer 112 may be a silicon dioxide layer 500 to 1000 nmthick formed by PECVD using tetraethyl orthosilicate, also known astetraethoxysilane (TEOS). An optional second etch stop layer 116 may beformed over the dielectric layer 114. The second etch stop layer 116 maybe primarily a silicon nitride-based dielectric material, 50 nanometersto 150 nanometers thick, and may be formed to provide an etch stop forsubsequent etching steps.

A magnetic core 120 is formed on top of the underlying dielectric layer110. The magnetic core 120 material is a multilayered stack ofalternating layers of NiFe permalloy (NiFe) which is a material withhigh magnetic permeability and low resistance, and AlN which is adielectric. In an example fluxgate magnetometer 111 the magnetic core iscomprised of 3 to 10 layers of NiFe and AlN wherein the NiFe layers andthe AlN layers alternate and wherein the NiFe layers have a thickness ofabout 225 nm to 425 nm and the AlN layers have a thickness of about 5 nmto 15 nm.

A second ILD layer 124 is disposed over the underlying dielectric layer110 and over the sides and top of the magnetic core 120. The thicknessof the second ILD layer 124 depends upon the thickness of the magneticcore 120. The thickness of the second ILD layer 124 may have a thicknessbetween about 1 micron and 4 microns depending upon the thickness of themagnetic core 120. In an example fluxgate magnetometer the thickness ofthe magnetic core is about 1.4 microns and the thickness of the secondILD layer is about 3.5 microns of silicon dioxide deposited using aPECVD TEOS process.

A plurality of second vias 132 having copper damascene structures aredisposed in the second ILD layer 124. Some of the second vias 132 extendthrough the underlying dielectric layer 110 and make connections to thefirst metal lines 104. The second vias 132 may be part of dual damascenestructures which include second metal lines 126 over the second vias132, as depicted in FIG. 1. The integrated circuit 100 may include anprotective overcoat layer 134 disposed over the second ILD layer 124 andover the second metal lines 126 with a bond pad opening 136 for makingelectrical connection as shown in FIG. 1. Alternatively the integratedcircuit may include a third etch stop layer disposed over the second ILDlayer 124 and second metal lines 126 and possibly a third ILD layer overthe third etch stop layer. Additional layers of ILD and interconnect maybe formed between the second metal lines and a bond pad opening 136.

The second metal leads 130 above the magnetic core 120 are connected tothe first metal leads 108 under the magnetic core 120 by a first set ofvias 132 disposed in front of the magnetic core 120 and by a second setof vias 132 disposed behind the magnetic core 120. These vias 132connect the first metal leads 108 to the second metal leads 130 to forma coil 212 (in FIG. 2) which winds around the magnetic core 214. Thefirst metal leads 108 may be connected to the second metal leads 130 toform more than one coil surrounding the magnetic core 120. The coils areelectrically isolated from the magnetic core 120 and the second vias 132by the underlying first dielectric 110 and by the second ILD layer 124.

As is additionally illustrated in FIG. 2, first metal leads 108underlying the magnetic core 120 are connected to the second metal leads130 overlying the magnetic core 120 by vias 213 in front of the magneticcore 120 and by vias 217 behind the magnetic core 120. Although only onecoil 212 is shown, two or more coils are typically formed around themagnetic core 120 to form the fluxgate magnetometer 111.

FIG. 3A through FIG. 3D are cross sections of the integrated circuit ofFIG. 1 depicted in successive stages of fabrication.

Referring to FIG. 3A, the underlying dielectric layer 110 is formed overlower layers including a substrate of the integrated circuit 100. Theunderlying dielectric layer 110 may be formed of layers of differentdielectric materials. A first dielectric layer 112 in the underlyingdielectric layer stack 110 may be an etch stop layer 112 formed on firstILD layer 102 and on first metal leads 104. The first etch stop layer112 may be silicon nitride with a thickness of between about 35 nm and150 nm and may be formed by PECVD using silane, ammonia and nitrogengases, to provide desired etch selectivity to subsequently formedoverlying layers of silicon dioxide-based dielectric materials. Thefirst dielectric layer 112 also advantageously provides a diffusionbarrier to copper 106 in the underlying first metal leads 104.

The second dielectric layer 114 in the underlying dielectric stack 110may be a silicon dioxide-based dielectric material about 500 nm to about1000 nm thick formed by plasma enhanced chemical vapor deposition(PECVD) using tetraethyl orthosilicate, also known as tetraethoxysilane(TEOS), or other suitable process.

A third dielectric layer 116 in the underlying dielectric stack 110 maybe an optional second etch stop layer 116. The second etch stop layer116 may be silicon nitride with a thickness between about 35 nm and 150nm formed by PECVD using silane, ammonia and nitrogen gases, and mayprovide desired etch selectivity to a subsequent etch.

Referring to FIG. 3B a magnetic core material layer 308 is formed on theunderlying dielectric layer 110. The magnetic core material layer 308 iscomposed of alternating layers of NiFe and AlN. A NiFe layer has athickness of about 225 nm to 425 nm and a AlN layer has a thickness ofabout 5 nm to 15 nm. In an example embodiment the NiFe layer thicknessis about 325 nm and the AlN layer thickness is about 10 nm. In theexample embodiment the magnetic core is a stack of about 3 to 10 layersof AlN/NiFe. The AlN layers interposed between the NiFe layers improveperformance of the fluxgate magnetometer by reducing losses due to eddycurrents especially at high frequencies. A magnetic core pattern 310 isformed over the magnetic core material layer 308 and exposes themagnetic core material layer 308 where it is to be etched away. Themagnetic core pattern 310 may include photoresist formed by aphotolithographic process, and may possibly include an anti-reflectionlayer and/or a hard mask layer.

Referring to FIG. 3C, the magnetic core material layer 308 is etchedfrom the regions exposed by the magnetic core pattern 310 to form themagnetic core 120.

A wet etchant which etches the stack of magnetic core material 308resulting in good dimensional control and a good profile containsphosphoric acid, acetic acid, nitric acid, and deionized (DI) water. Thewet etchant is composed of between about 20-40 wt % concentratedphosphoric acid, between about 1-10 wt % concentrated acetic acid,between about 0.1% and 3% concentrated nitride acid, and between about20 and 80% wt % DI water. The wet etchant may be used in a temperaturerange of about 20° C. to 35° C. Etching time depends upon thetemperature. The etch rate is faster at higher temperatures so etch timeis shorter at higher temperatures. The preferred composition of the wetetchant depends upon the relative thickness of the NiFe and AlN layers.In an example fluxgate magnetometer the wet etchant is about 30% byweight phosphoric acid, about 4% by weight acetic acid and about 0.45%by weight nitric acid. This etchant etches the NiFe and the AN atapproximately the same rate to provide a magnetic core with a goodprofile. Unlike other etchants which typically etch the NiFe and AlN atdifferent rates resulting in a profile where the AlN layers protrudebeyond the NiFe layers, this etchant results in a magnetic core profilewhere the edges of the AlN layers and the NiFe layers are substantiallycollinear. A smooth profile provides improved mechanical stabilitybetween the dielectric that overlies the magnetic core thus avoidingdelamination that may result in circuit failure. The smooth profile alsoreduces noise which might limit the sensitivity of the magnetic core inthe detection of weak magnetic fields.

Referring to FIG. 3D, after the magnetic core pattern 310 is removed,fabrication of the integrated circuit 100 is continued to provide thefluxgate magnetometer 111 structure of FIG. 1.

When the magnetic core material stack 308 is thick, the etch rate of themagnetic core material may start slowing when etching time exceeds about4 minutes. The etch rate may be restored by performing a DI rinse andthen returning the wafers to the wet etchant bath such as is describedin the process flow of FIG. 4.

Referring to FIG. 4 the wafers with the patterned NiFe/AlN magnetic corematerial are put in the etchant bath described above in step 402 andetched for a time of less than about 6 minutes in step 404. In anexample process the wafers are etched for about 4 minutes.

In step 406 the wafers are rinsed with DI water and in step 408 thewafers are checked to see if the NiFe/AlN magnetic core material iscleared from the regions exposed by the magnetic core pattern. If it isclear, the wafers are moved onto the next process step 410 in theprocess flow.

If the NiFe/AlN magnetic core material is not clear, the wafers arereturned to the etching bath 402 and steps 402, 404, and 408 arerepeated until the magnetic core material is etched clear.

Referring to FIG. 5, encapsulation of the magnetic core 120 with anunderlying stress relief layer 118 and/or an overlying stress relieflayer 122 may improve yield by eliminating delamination of the magneticcore 120 from the surrounding dielectric layers 110 and 124 due tostress.

FIG. 6A through FIG. 6C are cross sections of the integrated circuit ofFIG. 5 depicted in successive stages of fabrication.

Referring to FIG. 6A a first layer of stress relief material layer 606such as titanium is deposed on etch stop layer 116. Other stress reliefmaterials such as Ta, TiN, TaN, Ru, and Pt may be used. Titanium is usedfor illustration. For this embodiment etch stop layer 116 is notoptional. It provides etch stop selectivity to a subsequent plasma etchcontaining fluorine. The titanium layer 606 may be deposited usingphysical vapor deposition (PVD) such as sputtering to a thicknessbetween about 30 nm to 50 nm.

The magnetic core material layer 608 is formed on the stress reliefmaterial layer 606 as described previously. A magnetic core pattern 610is formed on the magnetic core material layer 608.

Referring to FIG. 6B the wet etchant is used to etch away the magneticcore material 608 where exposed by the magnetic core pattern 610. Theamount of undercut 605 of the magnetic core pattern 610 at the bottom ofthe magnetic core 120 may be controlled with the wet etch over etchtime. It is advantageous for the stress relief layer 606 to extendbeyond the magnetic core 120 by at least about 1.5 microns to providesufficient mechanical stability to eliminate delamination that mightlead to circuit failure. The wafers may remain in the wet etchant bathso that the magnetic core pattern extends a length 605 of at least 1.5microns beyond the bottom of the magnetic core 120. This enables thephoto mask that forms the magnetic core pattern 610 to be reused to forma stress relief material etch pattern 614 saving considerable cost.

Referring now to FIG. 6C the magnetic core pattern 610 is removed and asecond layer of stress relief material 612 such as titanium may beformed over the first stress relief material layer 606 and over the topand sides of the magnetic core 120. The titanium layer 612 may bedeposited using physical vapor deposition (PVD) such as sputtering to athickness between about 90 nm to 300 nm.

A stress relief material etch pattern 614 is formed on the second stressrelief layer 612. The same photomask that was used to form the magneticcore pattern 610 may be used to form stress relief material etch pattern614. The stress relief material etch pattern 614 extends a length 615 ofat least 1.5 microns 615 past the base of the magnetic core 120.

The second titanium layer 612 and the first titanium layer 606 exposedby the stress relief material etch pattern 614 are etched using a plasmaetch containing fluorine gas to form the structure in FIG. 5. The plasmaetch stops on etch stop layer 116. Extending the first and second stressrelief layers at least 1.5 microns beyond the edges of the magnetic coreprovides sufficient mechanical stability to prevent delamination of themagnetic core 120 from the surrounding dielectric layers 116 and 124which may introduce noise reducing the sensitivity of the fluxmagnetometer and also may result in circuit failure.

The fabrication of the integrated circuit 100 is then continued toprovide a fluxgate magnetometer structure 111 similar to that shown inFIG. 1 with the addition of stress relief encapsulation.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only and not limitation. Numerous changes to the disclosedembodiments can be made in accordance with the disclosure herein withoutdeparting from the spirit or scope of the invention. Thus, the breadthand scope of the present invention should not be limited by any of theabove described embodiments. Rather, the scope of the invention shouldbe defined in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A method of forming an integrated circuit,comprising the steps: forming a first dielectric layer on a wafer of theintegrated circuit; forming a layer of magnetic core material composedof alternating layers of NiFe permalloy and AlN dielectric on the firstdielectric layer; forming a magnetic core pattern on the magnetic corematerial which exposes areas outside a magnetic core area; etching witha wet etchant to remove the magnetic core material where exposed by themagnetic core pattern to form a magnetic core wherein the wet etchant iscomprised of phosphoric acid, acetic acid, nitric acid and deionizedwater; and removing the magnetic core pattern.
 2. The method of claim 1,wherein the phosphoric acid is concentrated phosphoric acid with aweight percent between 10% and 40%, the acetic acid is concentratedacetic acid with a weight percent between 1% and 10%, and the nitricacid is concentrated nitric acid with a weight percent between 0.1% and3%.
 3. The method of claim 1, wherein each layer of NiFe permalloy has athickness between 225 nm and 425 nm and where each layer of AlN has athickness between 5 nm and 15 nm and where there are between 3 and 10layers each of NiFe permalloy and AlN.
 4. The method of claim 1 whereineach layer of NiFe permalloy has a thickness of 325 nm and each layer ofAlN has a thickness of 10 nm.
 5. The method of claim 1, wherein the stepof etching with the wet etchant comprises repeated cycles of etching themagnetic core material with the wet etchant followed by a deionizedwater rinse until the magnetic material is removed from the exposedareas.
 6. The method of claim 1, wherein the step of etching with a wetetch comprises repeated cycles of etching the magnetic core materialwith the wet etchant for a time less than 6 minutes followed by adeionized water rinse until the magnetic material is removed from theexposed areas.
 7. The method of claim 1, wherein the wet etchant iscomprised of 30% by weight concentrated phosphoric acid, 4% by weightconcentrated acetic acid, 0.45% by weight concentrated nitride acid, anddeionized water.
 8. A method of forming an integrated circuit,comprising the steps: forming a first dielectric layer on a wafer of theintegrated circuit; forming an etch stop layer on the first dielectriclayer wherein the etch stop layer is silicon nitride; forming a firststress relief material layer on the etch stop layer; forming a layer ofmagnetic core material composed of alternating layers of NiFe permalloyand AlN dielectric on the stress relief material layer; forming amagnetic core pattern on the magnetic core material which exposes areasoutside a magnetic core and where the magnetic core pattern is formedusing a photomask; etching with a wet etchant to remove the magneticcore material where exposed by the magnetic core pattern to form themagnetic core wherein the wet etchant is comprised of phosphoric acid,acetic acid, nitric acid and deionized water; overetching the magneticcore material so that the magnetic core pattern extends beyond thebottom of the magnetic core by at least 1.5 um; removing the magneticcore pattern; forming a second stress relief material layer on the firststress relief material layer and on the top and sides of the magneticcore; forming a stress relief material etch pattern on the second stressrelief material wherein the stress relief material etch pattern extendsbeyond the bottom of the magnetic core by at least 1.5 um and whereinthe stress relief material etch pattern is formed using the samephotomask that is used to form the magnetic core pattern; etching thefirst and the second stress relief material layers using a plasma etchwith a fluorine containing gas wherein the plasma etch stops on the etchstop layer; and removing the stress relief material etch pattern.
 9. Themethod of claim 8, wherein the phosphoric acid is concentratedphosphoric acid with a weight percent between 10% and 40%, the aceticacid is concentrated acetic acid with a weight percent between 1% and10%, and the nitric acid is concentrated nitric acid with a weightpercent between 0.1% and 3%.
 10. The method of claim 8, wherein the wetetchant comprises 30% by weight concentrated phosphoric acid, 4% byweight concentrated acetic acid, 0.45% concentrated nitric acid, and DIwater.
 11. The method of claim 8, wherein the first stress reliefmaterial has a thickness of between 30 nm and 50 nm and where the secondstress relief layer has a thickness between 90 nm and 300 nm.
 12. Themethod of claim 8, wherein each layer of NiFe permalloy has a thicknessbetween 225 nm and 425 nm and where each layer of AlN has a thicknessbetween 5 nm and 15 nm and where there are between 3 and 10 layers eachof NiFe permalloy and AlN.
 13. The method of claim 8, wherein the firstand second stress relief layers are selected from the group consistingof Ti, TiN, Ta, TaN, Ru, and Pt.
 14. The method of claim 8, wherein thefirst stress relief layer is titanium with a thickness between 30 nm and50 nm and wherein the second stress relief layer is titanium with athickness between 90 nm and 300 nm.
 15. The method of claim 8, whereinthe step of etching with the wet etchant comprises repeated cycles ofetching the magnetic core material with the wet etchant followed by adeionized water rinse until the magnetic material is removed from theexposed areas.
 16. The method of claim 8, wherein the step of etchingwith a wet etch comprises repeated cycles of etching the magnetic corematerial with the wet etchant for a time less than 6 minutes followed bya deionized water rinse until the magnetic material is removed from theexposed areas.
 17. The method of claim 8, wherein the etch stop layer issilicon nitride formed by PECVD using silane, ammonia, and nitrogengases with a thickness between 35 nm and 150 nm.